Heavy-chain class switching refers to the process in which a clone of B lymphoid cells first expresses a single heavy chain variables (V) region in association with mu heavy chains, and, subsequently, in association with another heavy chain class such as gamma or alpha. Most previous studies of this process have compared heavy-chain gene structure in unrelated tumor cell lines which secreted different classes of immunoglobulin chains. The research has developed Abelson murine leukemia virus-transformed pre-B cell lines which, in culture, undergo all of the gene reorganization events associated with the differentiation of cells of the B-lymphoid (antibody-producing) lineage including heavy-\and light-chain variable region assembly and heavy-chain class switching. The analyses of several such lines which have undergone a class switch event (mu to gamma2b) in culture, indicated that they have employed the genetic recombination/deletion mechanism thought to mediate physiological class switch events. Therefore, these lines offer an ideal model system to study the dynamic aspects of class switching. It is proposed to extend studies of this phenomenon by comparative analysis of class switch events occurring between the endogenous immunoglobulin gene segments in these lines and potential recombination events occurring within appropriate class switch recombination substrates that have been introduced into the genome of the same lines. These studies should help to answer a variety of unresolved questions concerning the class switch phenomenon including the details of the molecular mechanism, the differentiation stage at which switching occurs, and the mechanisms controlling the process. Some of the A-MuLV transformants have recently been shown to undergo a high rate of somatic mutation. Additional gene transfer studies will exploit such lines to elucidate the mechanism and control of the somatic mutation process. Finally, gene transfer technology will be used to analyze the well-established phenomenon of cellular oncogene (c-myc) translocation into the class switch region of the immunoglobulin heavy-chain genes in tumors of the B-cell lineage. These studies should help to elucidate the molecular mechanism of this translocation as well as the role of the class switch recombinase system in this aberrant translocation process. (HI)